Multiple myeloma (MM) is the second most commonly diagnosed hematologic malignancy with an estimated 20,520 newly diagnosed cases and 10,610 deaths due to MM in the United States in 2011. MM is a disease of the elderly with a median age at diagnosis of 70 years and 37% of patients older than 75 years. The number of geriatric MM patients is expected to increase over time because of the increasing life expectancy of the normal population. MM is a malignancy of clonal plasma cells in the bone marrow (BM) characterized by the presence of paraproteinemia, destructive bone disease, hypercalcemia, renal failure, and/or hematological dysfunction. Although survival rates of MM patients have improved by recent therapeutic advances, MM remains incurable due to the persistence of minimal residual disease. Thus, novel modalities complementing or improving current treatment options are desperately needed. There is ample evidence that immunomodulatory drugs are effective against MM. Thus, the use of a potent immunotherapeutic is an attractive approach to provide durable immune responses to or even potentially cure patients with MM. IL-15, a necessary factor for the development, proliferation and activation of effector NK cells and CD8* memory T cells, exhibits potent anti-tumor activities against well-established tumors in laboratory animal models and is listed by a recent NCI review as the most promising product candidate among twelve immunotherapy drugs that could potentially cure cancer. We have previously reported the isolation of a novel IL-15 mutant with a 4-fold increase in biological activity. The pharmacokinetics and biological activity of this superagonistic IL-15 (IL-15N72D) have been further improved by creating a complex with an IL-15 receptor - IgG1 fusion protein (IL-15R/Fc). We postulate that that intravenous administration of the IL-15N72D:IL- 15R/Fc fusion complex will provide a durable, potent and broad cell-mediated immune response, which would result in a highly efficacious and potentially curative immunotherapy for the treatment of patients with MM. This is supported by the data from our recent studies, indicating that this IL-15 fusion complex indeed eradicated well-established 5T33 myeloma tumors in an immunocompetent C57BL/6 mouse model. More importantly, short-term treatment provided long-lasting immunological effects that completely protected mice against subsequent tumor cell rechallenge. In this study described here, we propose to carry out experiments to reveal the mechanisms of action and to further evaluate the durability (a hallmark of immunotherapy) of IL- 15 superagonist-induced responses for 5T33 tumor-based myeloma in mice. These efforts will pave the way for pre-clinical studies, which will include additional efficacy studies to determine optimal dosing and pharmacokinetics and toxicology evaluation of the complex, as part of the SBIR Phase II project, to support clinical development. Our ultimate goal is to enter the IND phase and to conduct clinical trials using the IL- 15N72D:IL-15R/Fc superagonist complex to treat patients with MM. PUBLIC HEALTH RELEVANCE: In this proposal, we propose to carry out experiments to reveal the mechanisms-of-action and to evaluate the anti-tumor durability (a hallmark of immunotherapy) of a novel interleukin-15 (IL-15) superagonist complex (IL- 15N72D:IL-15R/Fc) in the 5T33 tumor-based multiple myeloma model in mice. Positive outcomes from this proposed study would provide justification for clinical development of this complex as a safe, durable, potent and cell-mediated immunotherapy to treat patients with multiple myeloma.